RF and Microwave Spectrum Analyzers are and have long been an important instrument family, used in a variety of applications including Laboratory analysis of signals and modulation spectra, Telecommunications signals, Satellite Communications, Electronic Warfare and other Defense applications. Recently, due to the increased use in WiFi applications such as 3G, 4G, and LTE architectures, it has become very important to have handheld spectrum analyzers that are light weight, have long battery life, and still have a high degree of accuracy. In addition the ability to quickly identify and analyze received signals has become very critical in many defense applications.
Most spectrum analyzers are built using a heterodyne type design. This is shown in block diagram form in FIG. 1. The incoming signal 101, whose frequency and other spectral characteristics are unknown, is mixed or frequency converted in a series of mixers 104 and 107 in this figure (or more or fewer mixers as desired by the designer of the analyzer) with a series of local oscillators 103 and 106, the first of which, 103 in FIG. 1, typically, is a swept signal source that is capable of being frequency synthesized to a desired level of frequency accuracy and stability. The mixer 104 output of this First Local Oscillator also called First LO, is filtered through a fixed bandwidth bandpass filter 105 to produce a First Intermediate Frequency, or First IF. Assuming that the purity of the First LO is much better than that of the incoming signal, any input to the signal analyzer will produce a signal with the same spectral characteristics as the input signal at the First IF at frequencies given byFif=Absolute value of(±m·Flo±n·Fsig)
Where Fsig is the frequency being scanned for the existence of a signal, and m and n are integers. Typically all IF frequencies other than the desired one are eliminated by means of the First IF filter 105. In many analyzers m=1, but m can be 1,2, or a higher number depending upon the harmonic of the LO that the signal will mix with to produce Fif. Both m·Flo+Fsig and m·Flo−Fsig will create identical outputs at the input to the First IF. However, only one will be equal to Fif, and this signal will pass to the second W. After the First IF, a series of mixers, in this Figure, 107 is utilized in order to more accurately filter the First IF and to make it possible to detect, measure 109 and display 110 the resultant translated form of the input signal with the maximum amount of fidelity to the actual input signal as possible. This set of filter bandwidths is often called the Resolution Bandwidth of the analyzer, and it can be varied from front panel or remote controls. Unfortunately, the creation and existence of multiple harmonics of the signal and LO frequency within the mixer implies that there is the mathematical possibility that for some other values of n and m, n·Fsig mixing with m·Flo creates a signal at Fif. In this case, this signal will pass through as if it were a legitimate signal. This is called an image response. This is not acceptable as it can produce a spurious signal display at a frequency where there is no actual signal present at the input. In order to eliminate this, the input signal is passed through a tunable filter 102 that tracks the first LO frequency with an offset equal to the First IF frequency such that only the desired signal can cause the desired IF. This is called a Preselector, and in broadband analyzers the function is often realized by a tunable bandpass filter called a YIG tuned Filter (YTF). The LO is often a Yig Tuned Oscillator (YTO). If the YTF is integrated with the mixer, as is often done, such a component is called a Yig Tune Mixer or YTX.
There are many advantages to this traditional approach, the most important of which is that, until this present invention there always was needed a preselector, and there was no easy way other than a YIG device to achieve a broadband tunable Preselector Filter that tracked in frequency with a Local Oscillator. YIGs depend upon the resonance of Yttrium Iron Garnet spheres in the presence of a magnetic field. This resonance is almost linear with the applied magnetic field, so that if the magnetic field is realized by means of an electromagnet, the resonator can be frequency tuned over a wide frequency range in a linear relation to its drive voltage or current. A typical YIG structure is shown in FIG. 2, where the coil 201 tunes the sphere 202 via the electromagnet 206, creating a tuned resonance in coupling RF loops 203 and 204 (in this example a filter) that is the basis of the tuning of the filter in the YTF. When an oscillator is needed, only one coupling loop, for example 203 is used as a resonator for the local oscillator, YTO. In a YTF, at the resonance frequency if the loops are orthogonal, the RF energy is coupled into the sphere and transferred to the output loop. At all other frequencies there is no energy transfer except for leakage. The advantage of YIG resonance is that it has a high Quality Factor, called Q, which is a measure of purity of resonance, which allows for very good signal selectivity. The disadvantages unfortunately are many. The electromagnets have significant hysteresis which needs to be corrected. Their linearity is not perfect. They suffer from significant thermal drift. They require very high voltages to allow the magnets to overcome the tuning coil IR (current, 1 times Resistance R) voltage drops, and the L·(di/dt) drop, where L is the coil inductance and di/dt is the rate of change of current with time. This severely limits the speed with which the YIG devices can be tuned to different frequencies. In a spectrum analyzer, where the LO and preselector filter need to exactly track each other within the first IF bandwidth, these drifts impose significant other constraints. Another very difficult constraint is that while the YTO can indeed be phase locked or synthesized to any accuracy its crystal reference, 113, in FIG. 1, provides, the YTF, being essentially open loop, cannot have such accuracy. As a result, in YIG tuned spectrum analyzers, except for the start of a sweep, the first LO is never phase locked or accurate as a locked oscillator. The sweeps are essentially analog sweeps and the measurements at any frequency are subject to this analog inaccuracy. As a result, often, to narrow down on a signal the instruments need to follow a sequence of Peak Search, selecting the maximum value of the signal display—Centering the Frequency Band around the Peak—then reducing the span has to be done many times. Thus for each reduction of resolution bandwidth a minimum of 3 sweep are needed. To narrow down a signal from a 10 GHz span to a 100 KHz span, often 30 or more sweeps, each taking up 50 to 400 milliseconds are needed so that any measurement that requires this accuracy may need 5 to 10 seconds.
Other methods have been proposed and tried to solve these problems. It is possible for example, and has been proposed, to build the preselector out of a bank of switched bandpass filters. The problem with this solution is that if one wanted to build a broadband analyzer covering many decades of frequency range, many microwave filters would be required, along with banks of microwave switches. This is a very expensive and almost impractical solution. As a result this is not a method that is used except in specific narrow bandwidth instruments for very specialized applications. Another method that is used in many fixed or narrow frequency range receivers is called the Zero-IF receiver. Here, the mixer output is filtered with a very low frequency low pass filter, and when the LO frequency is equal to the input frequency there is a DC output voltage. There are no extra stages of IF. The problem with this method is that the IF is really a DC or close to DC value, which requires to sweep the signal very slowly, and is not usable even in moderate bandwidth receivers. Zero IF receivers are used, and are very effective in fixed frequency receivers, for example in testing known communication channels. They are very impractical in broadband receivers.
Another problem with these traditional approaches is that, to identify the existence of signals in the frequency range of interest and then to analyze the modulation, phase noise, or other characteristics of the spectra, multiple sweeps are needed. Since, due to YIG tuning speed limitations, each sweep is typically several hundred milliseconds in duration, any measurement that takes multiple sweeps will take many seconds to complete. In an agile signal environment for example in Electronic Warfare this is not be acceptable.
The present invention addresses these issues. It uses a completely different architecture to eliminate the need for preselection, so that accurate signal analysis can occur in microseconds, 100 to 1000 times faster than existing methods.